1. Technical Field of the Invention
The present invention relates generally to a battery control method for hybrid vehicles, and more particularly to a state-of-charge determining method of determining a state-of-charge of a storage battery installed in a hybrid vehicle accurately.
2. Background Art
In recent years, attention is paid to hybrid vehicles equipped with, as a power source, an internal combustion engine and an electric motor actuated by a storage battery for fuel economy. Typical batteries installed in the hybrid vehicles are discharged during a high-load operation such as acceleration and recharged during a low-load operation such as traveling at constant speed or deceleration. In order to carry out such discharging and recharging effectively, it is important to keep SOC (State Of Charge which is also referred to as an available reserve capacity or residual electric energy) at a middle value (e.g., 50% to 70% of a fully charged energy of a battery). It is, thus, essential to monitor the SOC of the storage battery.
As a method of measuring the SOC, there is known a technique for integrating or totalizing the amount of current discharged from a storage battery. This technique, however, encounters a drawback in that errors in totalizing the discharged current are accumulated due to a variation in charging/discharging efficiency, thus making it difficult to measure the amount of reserve current in the battery accurately. In order to avoid this problem, Japanese Patent First Publication No. 2000-69606 proposes a correction system designed to correct the charging/discharging efficiency as a function of a difference between an actual state of charge and an estimated state of charge of a storage battery. The actual state of charge is determined by an upper or a lower limit of a voltage-to-current characteristic stored in a memory of the system when it is reached. The estimated state of charge is determined by a totalized amount of current discharged from the battery. The system, however, has a problem in that it is difficult to eliminate the totalizing error completely because of a change in charging/discharging efficiency arising from the history of use of the battery.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a battery control method for hybrid vehicles which is capable of controlling a state-of-charge of a storage battery to bring it into agreement with a target one accurately.
According to one aspect of the invention, there is provided a battery control method of charging or discharging a storage battery installed in a hybrid vehicle to bring a calculated state-of-charge of the storage battery into agreement with a target state-of-charge. The method comprises the steps of: (a) determining an open-circuit voltage of the storage battery installed in the hybrid vehicle using a voltage developed across the storage battery and a current flowing into or out of the storage battery in a given voltage-to-current relation of the storage battery; (b) totalizing an amount of current flowing into or out of the storage battery in a cycle to calculate a state-of-charge of the storage battery; (c) comparing the open-circuit voltage with a reference voltage which corresponds to a reference state-of-charge predetermined in a state-of-charge correction range defined around a target state-of-charge of the storage battery; and (d) correcting the state-of-charge of the storage battery when the state-of-charge lies within the state-of-charge correction range by brining the state-of-charge close to the reference state-of-charge based on a result of comparison between the open-circuit voltage and the reference voltage. This eliminates a cumulative error in calculating the state-of-charge of the battery caused by a variation in charge/discharge efficiency of the battery. The reference voltage may be set to an open-circuit voltage corresponding to a central value of the state-of-charge correction range. The state-of-charge correction is prohibited when the state-of-charge changes out of the state-of-charge correction range, thereby minimizing an error in correcting the state-of-charge arising from use of a value of the open-circuit voltage undergoing the effect of the charge-caused polarization greatly.
In the preferred mode of the invention, the step is further provided which prohibits the correction step when it is determined that a discharge-caused polarization of the storage battery becomes greater than a given value, that is when the batter is charged so that the state-of-charge increases out of the state-of-charge correction range, after which the battery is discharged so that the state-of-charge returns and falls within the state-of-charge correction range, thereby eliminating an error in correcting the state-of-charge arising from the discharge-caused polarization of the battery.
The correcting step increases the state-of-charge when the open-circuit voltage is greater than the reference voltage and decreases the state-of-charge when the open-circuit voltage is smaller than the reference voltage.
The method may further comprise the step of prohibiting the correction step for a preselected period of time when the storage battery has been discharged so that the state-of-charge decreases by a given amount out of the state-of-charge correction range.
The method may further comprise the step of determining whether the open-circuit voltage has exceeded a high open-circuit voltage higher than the reference voltage or not and the step of correcting the reference voltage by a preselected value when it is determined that the open-circuit voltage has exceeded the high open-circuit voltage.
In instead of the open-circuit voltage, a voltage developed across terminals of the storage battery when the storage battery is charged or discharged with a given current may be used.
The correction step changes the state-of-charge of the storage battery by a preselected correction value in a cycle when the state-of-charge lies within the state-of-charge correction range. The method further comprises the step of determining that the storage battery has undergone a drop in state-of-charge due to self discharge when a cumulative value of the correction value exceeds a given value, and the open-circuit voltage remains below a given voltage lower than the reference voltage.
According to the second aspect of the invention, there is provided a battery control method of charging or discharging a storage battery installed in a hybrid vehicle to bring a calculated state-of-charge of the storage battery into agreement with a target state-of-charge. The method comprises the steps of: (a) determining an open-circuit voltage of the storage battery installed in the hybrid vehicle using a voltage developed across the storage battery and a current flowing into or out of the storage battery in a given voltage-to-current relation of the storage battery; (b) totalizing an amount of current flowing into or out of the storage battery in a cycle to calculate a state-of-charge of the storage battery; (c) determining whether the storage battery has been charged so that the state-of-charge of the storage battery increases out of a preselected state-of-charge range defined around the target state-of-charge or not; (d) determining whether the open-circuit voltage has reached a high open-circuit voltage higher than a reference voltage which corresponds to a reference state-of-charge predetermined in the state-of-charge range or not; and (e) discharging the storage battery until the state-of-charge reaches a given value smaller than the preselected state-of-charge range by a given amount when it is determined that the open-circuit voltage has reached the high open-circuit voltage.
In the preferred mode of the invention, the method further comprises the step of prohibiting the storage battery from being charged when it is determined that the open-circuit voltage has reached the high open-circuit voltage.
The method further comprises the step of, when the storage battery has been charged so that the state-of-charge is greater than a high state-of-charge higher than the preselected state-of-charge range, determining a maximum value of the state-of-charge calculated in a cycle and the step of discharging the storage battery by a given amount determined as a function of the maximum value of the state-of-charge after the storage battery is discharged, and the state-of-charge falls within the preselected state-of-charge range.
The maximum value may be reset each time the storage battery is discharged by the given amount.
The method further comprises the step of charging the storage battery until the open-circuit voltage reaches the high open-circuit voltage when the open-circuit voltage remains below the high open-circuit voltage for a given period of time.
Instead of the open-circuit voltage, a voltage developed across terminals of the storage battery when the storage battery is charged or discharged with a given current may be used.
According to the third aspect of the invention, there is provided a battery control method of charging or discharging a storage battery installed in a hybrid vehicle to bring a calculated state-of-charge of the storage battery into agreement with a target state-of-charge. The method comprises the steps of: (a) totalizing an amount of current flowing into or out of the storage battery in a cycle to calculate a state-of-charge of the storage battery; (b) determining whether the storage battery has been discharged until the state-of-charge of the storage battery decreases by a given amount out of a preselected state-of-charge range defined around the target state-of-charge or not; and (c) charging the storage battery until the state-of-charge reaches a given value higher than the preselected state-of-charge range by a predetermined amount when it is determined that the storage battery has been discharged until the state-of-charge of the storage battery decreases by the given amount.
The method further comprises the step of prohibiting the storage battery from being discharged further when it is determined that the storage battery has been discharged until the state-of-charge of the storage battery decreases by the given amount.
The method further comprises the steps of: determining an open-circuit voltage of the storage battery using a voltage developed across the storage battery and a current flowing into or out of the storage battery in a given voltage-to-current relation of the storage battery; comparing the open-circuit voltage with a reference voltage which corresponds to a reference state-of-charge predetermined in the preselected state-of-charge range; correcting the state-of-charge of the storage battery only when the state-of-charge lies within the preselected state-of-charge range by brining the state-of-charge close to the reference state-of-charge based on a result of comparison between the open-circuit voltage and the reference voltage; and prohibiting the correction step for a preselected period of time when the storage battery has been discharged so that the state-of-charge decreases by a given amount out of the preselected. state-of-charge range.
Instead of the open-circuit voltage, a voltage developed across terminals of the storage battery when the storage battery is charged or discharged with a given current may be used
According to the fourth aspect of the invention, there is provided a battery control method of charging or discharging a storage battery installed in a hybrid vehicle to bring a calculated state-of-charge of the storage battery into agreement with a target state-of-charge. The method comprises the steps of: (a) determining an open-circuit voltage of the storage battery installed in the hybrid vehicle using a voltage developed across the storage battery and a current flowing into or out of the storage battery in a given voltage-to-current relation of the storage battery; (b) totalizing an amount of current flowing into or out of the storage battery in a cycle to calculate a state-of-charge of the storage battery; (c) determining whether the storage battery has been charged so that the state-of-charge of the storage battery increases out of a preselected state-of-charge range defined around the target state-of-charge or not; (d) determining whether the open-circuit voltage has exceeded a high open-circuit voltage higher than a reference voltage which corresponds to a reference state-of-charge predetermined in the preselected state-of-charge range or not; and (e) correcting the reference voltage by a preselected value when it is determined that the open-circuit voltage has exceeded the high open-circuit voltage.
In the preferred mode of the invention, the method further comprises the step of determining whether the state-of-charge has exceeded a high state-of-charge higher than the preselected state-of-charge range or not. The correction step corrects the reference voltage when it is determined that the state-of-charge has exceeded the high state-of-charge.
The method further comprises the step of charging the storage battery until the open-circuit voltage reaches the high open-circuit voltage when the open-circuit voltage remains below the high open-circuit voltage for a given period of time.
Instead of the open-circuit voltage, a voltage developed across terminals of the storage battery when the storage battery is charged or discharged with a given current may be used.